The micro-mechanics of single molecules studied with atomic force microscopy
The atomic force microscope (AFM) in its forcemeasuring mode is capable of effecting displacements on an angstrom scale (10 A = 1 nm) and measuring forces of a few piconewtons. Recent experiments have applied AFM techniques to study the mechanical properties of single biological polymers. These properties contribute to the function of many proteins exposed to mechanical strain, including components of the extracellular matrix (ECM). The forcebearing proteins of the ECM typically contain multiple tandem repeats of independently folded domains, a common feature of proteins with structural and mechanical roles. Polysaccharide moieties of adhesion glycoproteins such as the selectins are also subject to strain. Forceinduced extension of both types of molecules with the AFM results in conformational changes that could contribute to their mechanical function. The force—extension curve for amylose exhibits a transition in elasticity caused by the conversion of its glucopyranose rings from the chair to the boat conformation. Extension of multidomain proteins causes sequential unraveling of domains, resulting in a force—extension curve displaying a saw tooth pattern of peaks. The engineering of multimeric proteins consisting of repeats of identical domains has allowed detailed analysis of the mechanical properties of single protein domains. Repetitive extension and relaxation has enabled direct measurement of rates of domain unfolding and refolding. The combination of sitedirected mutagenesis with AFM can be used to elucidate the amino acid sequences that determine mechanical stability. The AFM thus offers a novel way to explore the mechanical functions of proteins and will be a useful tool for studying the micromechanics of exocytosis.